Previous investigations of the hormonal processes involved in the control of aggression in mice have generally focused on the role of either the androgenic or estrogenic metabolites of testosterone (T) in the promotion of fighting behavior. However, this approach to the study of the hormonal regulation of aggression appears to be inadequate. This is because adult male mice are capable of responding to the aggression-promoting property of both androgen and estrogen while adult females will fight only when exposed to androgen; they are totally insensitive to the aggression-activating property of estrogen. These data suggest that a comprehensive model of the hormonal processes involved in the regulation of aggression in mice can be developed only if the specific effects of androgen and estrogen on the development and display of fighting behavior are defined. This will be the goal of the proposed series of experiments. Specifically, it will be shown that two biochemically independent aggression-activating systems, one androgen-sensitive, the other estrogen-sensitive, can be identified in males while only one such system, mediated by androgen, is found in females. Further, the contribution of neonatal androgen and estrogen stimulation in the establishment of each of these systems will be described, and the possible ways in which the systems might synergize to induce fighting will be explored.